1) Field of the Invention
The present invention relates to a data compression program, a data compression method, and a data compression device for compressing data. In particular, the present invention relates to a data compression program, a data compression method, and a data compression device which are suitable for compression of data to be transmitted.
2) Description of the Related Art
In recent years, data communications utilizing mobile telephone systems such as portable telephone or PHS (personal handyphone system) systems are extensively used. In particular, recently, data communication services using packet communications have become available, and many users are using such data communication services. According to the billing systems in the conventional packet communication systems, charges are calculated based on the numbers of packets which are transmitted and received. Therefore, it is desired that transmitted data are compressed as much as possible.
FIG. 13 is a conceptual diagram illustrating the conventional data compression processing. FIG. 13 illustrates data compression processing in an exemplary case where data output from a web server 910 and a mail server 920 are compressed by a compression processing unit 930, and transmitted through a network 940. For example, the web server 910 outputs document data 91 having an HTML (Hyper Text Markup Language) form, image data 92 compressed in a GIF (Graphics Interchange Format) form or the like, and other data, and the mail server 920 outputs mail data 93 having a text form. The document data 91, the image data 92, and the mail data 93 are compressed by the compression processing unit 930. Thus, document data 91a, image data 92a, and mail data 93a in compressed forms are generated in accordance with the compression method in the compression processing unit 930, and transmitted through the network 940.
According to the above construction, the volume of transmitted data can be reduced. Therefore, it is possible to reduce charges billed for packet communication as well as data transfer times.
In the above construction, in advance, each system (i.e., a device in which the compression processing unit 930 is installed) is configured to compress or not to compress data in the compression processing unit 930. That is, in advance, the compression processing function is set to “ON” or “OFF.” Therefore, when the compression processing unit 930 is configured to compress data to be transmitted, compression processing is performed on all data transmitted through the compression processing unit 930.
However, the volumes of some types of data to be transmitted are not substantially reduced. For example, the volumes of data which have already been compressed do not decrease even when the data are recompressed. In such a case, the useless attempt to compress the data causes performance loss.